1. Field of the Invention
The present invention relates to a magnetic memory device, and in particular, to a magnetic memory device that uses magnetoresistive elements as memory cells.
2. Description of the Related Art
In recent years, a magnetic random access memory (MRAM) has been proposed which is a nonvolatile memory utilizing memory cells composed of variable resistance elements that can store data on the basis of a variation in electrical resistance. For example, in the MRAM, memory cells are composed of Magnetic Tunnel Junction (MTJ) elements.
The MTJ element comprises a magnetic recording layer (also referred to as a free layer) in which the direction of magnetization varies depending on an external magnetic field and a magnetization fixing layer (also referred to as a pinned layer) in which the direction of magnetization is fixed so as to remain unchanged. The MTJ element has two reference values depending on the relative relationship between the directions of magnetization in the free layer and in the pinned layer; the MTJ has a resistance value Rp when the directions are parallel and a resistance value Ra when they are antiparallel. Data is stored by associating the resistance value Rp with data “0”, while associating the resistance value Ra with data “1”.
Data is written to the MTJ element by passing at least a specified current through a corresponding bit and word lines passing through the memory cell so as to cross at right angles and controlling the direction of magnetization in the free layer of the MTJ element in accordance with the magnitude of a composite magnetic field resulting from the current. Data is read by reading a voltage value at a specified current or a current value at a specified voltage as the resistance value of the MTJ element.
The MTJ element has one end electrically connected to a bit line and the other end electrically connected to a selection transistor. That is, one selection transistor corresponds to one MTJ element. Thus, for example, the area of a cell is determined by a region for the selection transistor which is composed of a metal oxide semiconductor field effect transistor (MOSFET). This precludes MTJ elements from being densely arranged, thus preventing an increase in the degree of integration of the MRAM.
A related technique has been proposed which comprises an MRAM having a cell array structure suitable for an increase in the degree of integration of cells (see Jpn. Pat. Appln. KOKAI Application No. 2003-249072).